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Facile Fabrication of Silver Nanofin Array via Electroless Plating Kentaro Miyoshi,† Yoshitaka Aoki,‡ Toyoki Kunitake,‡ and Shigenori Fujikawa*,† InnoVatiVe Nanopatterning Research Laboratory and Topochemical Design Laboratory, Frontier Research System, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ReceiVed NoVember 11, 2007. In Final Form: December 18, 2007 The fabrication of metallic nanostructures is one of the main issues in nanotechnology. This article describes the fabrication of a silver nanofin array by combining microlithography, electroless plating, and an etching technique. Fabricated Ag nanofins have a high aspect ratio (height/width ) 10, width ) 60 nm, height ) 600 nm), and their widths and heights can be controlled by the period of electroless plating and the height of the original line pattern. An isolated Ag nanofin was proven to show metallic electrical conductivity. The current process provides a rapid and shape-designable fabrication method of metallic nanostructures.
Introduction The fabrication of metal nanostructures, including lines, wires, belts, walls, and rings, has attracted much attention in nanotechnology1-9 because they may provide unique properties that are not available with bulk metallic materials. In particular, metallic nanofins are attractive as basic components of nextgeneration electronic circuits in IC chips. For example, the fintype field effect transistor, so-called “FinFET”, has a fin-type gate arrangement consisting of metal and metal oxide that are perpendicular to the silicon wafer surface, though the layer of metal and metal oxide is horizontally stacked on the wafer in conventional FET. Using a perpendicular arrangement of the gate, the size reduction and the denser integration of a transistor are achievable, leading to an enhancement in chip performance. As a metallic circuit line, the high aspect ratio (height/width) is strongly desired because the resistance can be reduced by increasing the cross-sectional area of the line. Optical, electron, and X-ray lithography are well known in nanofabrication. Although optical lithography is a high-throughput process, down scaling of the pattern to the sub-100 nm regime is technically difficult. Electron beam lithography can fabricate sub-100 nm structures; however, it is based on a sequential writing system, resulting in low throughput, and it is extremely expensive. In addition, it is difficult to fabricate a metallic nanofin with a high aspect ratio by these conventional photolithography processes. Therefore, a facile approach is desired to fabricate a metal nanofin with a high aspect ratio. We previously reported the fabrication of metal oxide nanowalls by the wet nanocoating/dry etching technique.10 In this process,
a resist pattern fabricated by photolithography on a Si substrate was used as template, and the template was coated with a silica nanofilm by the surface sol-gel process (wet nanocoating). The top layer of the silica coat and the template resist were removed by dry etching to leave behind only side walls of the coating layer on the substrate. Wet-coating processes other than the surface sol-gel process may also be used for this purpose. Electroless plating (EP) is a solution-based coating method and can coat complicated and nonelectrically conductive templates with various metal layers.11-13 Thus, EP was used as the coating method of the template to fabricate metal nanostructures in previous work.14-18 In this article, we report the fabrication of a silver (Ag) nanofin pattern by using the EP method. Silver metal is particularly appealing as a nanofin material because it has high electrical conductivity and shows unique physical properties in the nanometer regime.6,19 Figure 1 shows the schematic procedure for the fabrication of a Ag nanofin array. First, the resist line pattern for the template was prepared on a Si wafer by photolithography (Figure 1(1)). For a uniform coating of the template surface with a Ag nanolayer, Ag nanoparticles are introduced onto the template surface as catalytic crystal nuclei, (Figure 1(2)), and the incorporation of a surface catalyst is denoted “surface catalysis” hereafter. After surface catalysis, EP of silver was carried out to form a Ag nanolayer on the template surface (Figure 1(3)). The Ar plasma was applied to the Ag-coated template to remove the Ag top coat (Figure 1(4)), and the resist template was finally removed by H2 plasma treatment (Figure 1(5)). Experimental Section
* Corresponding author. E-mail:
[email protected]. Tel: +81-48-462-1111. Fax: +81-48-462-5490. † Innovative Nanopatterning Research Laboratory. ‡ Topochemical Design Laboratory.
1. Preparation of the Template for Electroless Plating. The resist line pattern for the template was prepared by photolithography, as described in an earlier paper.10 The resist polymer (Tokyo Ohka
(1) Xia, Y.; Yang, P. AdV. Mater. 2003, 15, 351-352. (2) Menke, E. J.; Thompson, M. A.; Xiang, C.; Yang, L. C.; Penner, R. M. Nat. Mat. 2006, 5, 914-919. (3) Xu, Q.; Bao, J.; Capasso, F.; Whitesides, G. M. Angew. Chem., Int. Ed. 2006, 45, 3631-3635. (4) Shen, G.; Chen, D. J. Am. Chem. Soc. 2006, 128, 11762-11763. (5) Reetz, M. T.; Winter, M. J. Am. Chem. Soc. 1997, 119, 4539-4540. (6) Rieter, W. J.; Taylor, K. M. L.; An, H.; Lin, W.; Lin, W. J. Am. Chem. Soc. 2006, 128, 9024-9025. (7) Wiley, B. J.; Chen, Y.; McLellan, J. M.; Xiong, Y.; Li, Z.-Y.; Ginger, D.; Xia, Y. Nano Lett. 2007, 7, 1032-1036. (8) Law, M.; Zhang, X. F.; Yu, R.; Kuykendall, T.; Yang, P. Small 2005, 1, 858-865. (9) Hu, H.; Wang, H.; McCartney, M. R.; Smith, D. J. J. Appl. Phys. 2005, 97, 054305.
(10) Fujikawa, S.; Takaki, R.; Kunitake, T. Langmuir 2006, 22, 9057-9061. (11) Formanek, F.; Takeyasu, N.; Tanaka, T.; Chiyoda, K.; Isikawa, A.; Kawata, S. Appl. Phys. Lett. 2006, 88, 083110-083112. (12) Ishii, D.; Aoki, K. i.; Nakagawa, M.; Seki, T. Trans. Mater. Res. Soc. 2002, 27, 517-520. (13) Yokoshima, T.; Nakamura, S.; Kaneko, D.; Osaka, T.; Takefusa, S.; Tanaka, A. J. Electrochem. Soc. 2002, 149, C375-C382. (14) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920-1928. (15) Leo, M. D.; Pereira, F. C.; Moretto, L. M.; Scopece, P.; Polizzi, S.; Ugo, P. Chem. Mater. 2007, 19, 5955-5964. (16) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700-702. (17) Scopece, P.; Baker, L. A.; Ugo, P.; Martin, C. R. Nanotechnology 2006, 17, 3951-3956. (18) Manuela, D. L.; Alexander, K.; Paolo, U. Electroanalysis 2007, 19, 227. (19) Ghaemi, H. F.; Thio, T.; Grupp, D. E. Phys. ReV. B 1998, 58, 6779-6782.
10.1021/la703512w CCC: $40.75 © 2008 American Chemical Society Published on Web 03/01/2008
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Figure 1. Fabrication procedure of the Ag nanofin array.
Figure 2. Cross-sectional SEM images of each step in the experiment. The inset images in panels 2, 3, and 5 are magnified images.
Figure 3. Correlation of the electroless plating time and the film thickness. The plating time was varied from 5 to 30 min. The error bars indicate the distributions of the observed film thicknesses from SEM images.
Kogyo, TDUR-P015 PM) is based on poly(vinyl phenol) structures and contains unprotected phenol-hydroxyl groups (for which the detailed molecular structure is undocumented). The line template was treated with oxygen plasma (SAMCO - Compact Etcher FA-1, radio frequency 13.56 MHz, power 10 W, pressure ∼10 Pa) for 3 s at room temperature to oxidize the pattern surface. For surface catalysis, the substrate was dipped into an aqueous solution of silver nitrate (1 M) for 3 min to incorporate Ag ions into the template
surface, rinsed with ion-exchanged water for a few seconds, and dried by flushing with nitrogen gas. The substrate was then dipped into aqueous sodium borohydride (10 mM) for 1 min to reduce Ag ions to metallic silvers, following by rinsing and drying. These processes were repeated three times to supply Ag particles to the surface of the resist pattern. 2. Ag Electroless Plating and Anisotropic Etching. EP of silver was carried out according to Formanek’s report.11 Aqueous ammonia (0.2 M) was added dropwise to an aqueous silver nitrate (0.15 M) until brown precipitates were formed and then dissolved. We used 1.9 M glucose in a mixture of methanol/water (3:7 vol/vol) as the reductant. The EP reaction was started by adding the glucose solution to the silver nitrate solution. The template substrate was immersed in this mixture, and the reaction was stopped by washing the substrate with ion-exchanged water. The Ar plasma (Samco, RIE-10NR) was applied to the Ag-coated template for 1 min at room temperature to remove the Ag top coat. The plasma power, chamber pressure, and gas flow rate were adjusted at 300 W, 10 Pa, and 30 sccm, respectively. The resist template was finally removed by H2 plasma treatment (100 W, 10 Pa, and 30 sccm) for 30 min at room temperature. 3. Conductivity Measurement of the Ag Nanofin. An isolated Ag nanofin was also fabricated on the Si wafer with a thermally grown silicon dioxide surface (the thickness of oxide layer was ca.
Facile Fabrication of SilVer Nanofin Array
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Figure 4. Height control of the Ag nanofins. (a) Schematic representation of the height reduction process. (b-d) Cross sectional images of the template (upper) and the corresponding Ag fin (lower) formed from templates that were non-pre-etched and pre-etched for 2 and 4 min, respectively. 500 nm) by employing a line template that possesses a 5 mm line width. The conductivity was measured by the two-probe method using a potentiostat (Princeton Applied Research, PARSTAT2263). 4. Elemental Analysis of Nanofin. Scanning transmission electron microscopy-energy-dispersive X-ray analysis (STEM-EDX) was carried out at 200 kV for the sample after Ag EP to examine the elemental composition of the coated layer. Carbon and gold were deposited on the sample to avoid surface damage during focused ion beam etching (FIB) of the specimen for STEM-EDX analysis.
Results and Discussion 1. Fabrication of the Ag Nanofin Array. Surface morphologies of the substrate for each experimental step were examined by scanning electron microscopy (SEM, Hitachi S-5200, 5 kV acceleration voltage), as shown in Figure 2. The line template is 790 nm high and 410 nm wide (Figure 2(1)). The surface catalysis process did not affect the surface smoothness or the size of the template (Figure 2(2)). The Ag nanoparticles formed by this surface catalysis were not observed in this SEM image. However, the nanoparticle was found only in the line-template regime after the removal of the template by calcination, and nothing was observed after template removal when the Ag seeding process was omitted (data not shown). Therefore, we conclude that the Ag nanoparticle was formed only on the template surface. After the EP process was conducted for 5 min, the Ag nanolayer was uniformly formed only on the resist-template pattern. The catalytic Ag nanoparticle did not exist on the bare surface of the Si wafer between the line templates. The Ag coating layer was smooth, and its thickness was estimated to be about 53 nm (Figure 2(3)). The top layer of the Ag nanofilm was selectively removed by Ar plasma etching (Figure 2(4)), and then the resist template was reductively decomposed by H2 plasma treatment. These treatments produced a nanoline array on the substrate, as shown in Figure 2(5). The height and width of the nanofins are ca. 600 and 60 nm, respectively.
2. Elemental Analysis by STEM-EDX. The elemental mapping of the Ag coating layer was examined by STEM-EDX (Supporting Information, Figure S1). The EDX results indicate that Ag was specifically detected in the nanolayer on the resist template. The point elemental analysis of the deposited Ag nanolayer (at the yellow circle in S1) revealed that the Ag/O ratio was 33:1. Thus, it is clear that a uniform layer of Ag metal was formed on the template surface without the oxidation of Ag. It is interesting that the uniform Ag layer was not formed without Ag seeding for EP. Larger Ag particles were deposited on the pattern in the latter case. 3. Thickness Control of the Ag Nanolayer. The width of the Ag nanofin corresponds to the thickness of the coated-Ag layer, and its film thickness depends on the period of the EP process. The period of the EP process was varied to change the Ag layer thickness. This was measured at six different points in the cross section of the line, and its correlation with EP time is given in Figure 3. The white circles are the average of the observed thicknesses, and the distribution of the surface roughness is indicated by the error bar. Whereas the average layer thickness increases with the EP period during the initial 5-20 min, it becomes saturated after 30 min. Apparently, sufficient amounts of the Ag ion were not provided for the film growth in the final stage of the EP reaction because active Ag species were simultaneously consumed in the bulk solution. 4. Height Control of the Ag Nanofin. In our system, the height of the Ag nanofin should depend on the height of the resist template and the etching time. Prior to the EP process, the original template height of 790 nm could be reduced to 510 and 240 nm by prolonged O2 plasma pretreatment (power 10 W, pressure ∼10 Pa, room temperature) for 2 and 4 min, respectively. The results were shown in Figure 4. The sample without prolonged plasma treatment gave a height of 600 nm, which is lower than that of the template (Figure 4(b)) as a result of the overetching
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Figure 5. Examples of Ag nanocylinders fabricated from the hole template. Left and right images are the surface morphology of the template and the fabricated Ag nanocylinder array, respectively.
of the top layer by Ar plasma under the current plasma condition. The lowered templates with heights of 510 and 240 nm gave Ag nanofins of 420 and 190 nm in height, respectively ((Figure 4c,d). From these results, the height of the resulted fin is controllable by simply changing the template height and the top-layer etching conditions. 5. Fabrication of Metallic Cylinder Array. The metal nanocylindrical structures, such as rings and tubes, are very useful nanomaterials, and much effort has been devoted to their fabrication.16,20 The current process is also applicable to other template patterns, and the hole structures were used as a template for the fabrication of metallic cylinders. After the EP process, Ag nanocylinders that are 400 nm in height and 450 nm in diameter were obtained from the hole structure (Figure 5). Small refuses, apparently etching dusts, were formed around the nanostructures. Optimization of the etching conditions will be essential for reducing such etching dust. It is clear from these results that the size and shape of the metallic nanopattern can be designed by choosing appropriate templates. The template can be nonconductive, and complicated template structures that cannot be coated by the dry-deposition method may be used. 6. Conductive Measurement of a Single Ag Nanofin. To determine the electric conductivity of the Ag nanofin, we employed a wider-line template (line width 5 mm) on the surfaceoxidized Si wafer. The resulting Ag fins were separated from each other with a pitch of 5 mm and are addressable as a single fin. Thus, Ag paste was placed on both ends of the single fin to connect the potentiostat via gold wires. The continuous fin was observed on the Si wafer by SEM observation. The conductivities of three different samples were roughly estimated by the twoprobe method, and the I-V profile showed ohmic behavior. The average conductivity of three Ag nanofins was calculated to be about 1.3 × 107 S/m, which is close to the bulk Ag value (6.1 × 107 S/m). From these results, the fabricated nanofins were conductive. Generally speaking, the electroless plating process with grain growth of the metals and the plating layer may have a point connection structure among metal nanograins, if the plating is not sufficient. In such a case, the conductivity would be affected by the metallic contact of grains and could be changed in every experiment because the connection points of the metal grains could be formed and broken during the conductivity measurement. (20) Xu, Q.; Perez-Castillejos, R.; Li, Z.; Whitesides, G. M. Nano Lett. 2006, 6, 2163-2165.
However, the prepared nanofins gave a constant I-V slope in the repetition of the measurement and indicated sufficiently high conducitivity. Therefore, the Ag nanofin must be composed of uniformly fused polycrystals rather than point-connected nanograins. Similarly, the fusion of Au polycrystals was also observed in Au EP.15 In our approach, the minimum fin width for metallic conductivity depends on the size of the Ag grain. Although the thinnest width of the Ag nanofin fabricated by our process was 44 nm in this work, the fin width may be further reduced because the conceivable size of the Ag nanoparticle in our method is a few nanometers. The optimization of the experimental conditions is the next target for further size reduction of the line width.
Summary and Conclusions We succeeded in the fabrication of an electroconductive Ag nanofin array on a Si wafer. The width and the height of the Ag nanofin can be controllable by the EP time, the shape and size of the template, and the etching process. Our surface catalysis plays an important role in forming a uniform, ultrathin metallic layer, and this uniform coating is essential to reducing the fin width without losing electrical conductivity. Although the fin width achieved in this report is smaller than 50 nm, further size reduction would be possible by optimizing the conditions of surface catalysis and electroless plating. Electroless plating is a solution-based metallic coating and can coat complicated nanotemplates regardless of their electric conductivity, unlike electric plating and sputtering. Furthermore, other metal materials are readily employed in electroless plating. In our process, the size reduction and pattern design are completely separated, and sub-50 nm scale patterning of metals is now possible by taking advantage of pattern design in conventional lithography. Although we employed rather simple template shapes to confirm the feasibility of our approach, the sophistication of the template pattern is the next step. Pattern and material variety on the scale of
